RE: "76 is an average score, not a pan".
Yes, this is true.
However, if you were applying to med school, an average score would feel like a pan, because your point of reference is not an average score for the entire population, but rather an average score for people like you, who plan to apply for med school.
Likewise, here, a 76 is not a notably bad Metacritic, but is is perhaps a notably bad Metacritic for a product expected to sell certainly 5+ million and probably more like 10+ million, given the generally assumed link between product quality (for which Metacritic is a reasonable proxy, which is why companies care about it) and WOM sales spread over time.
On the upside, basically every major current-gen release has gotten unexpectedly low review scores--Destiny mid-70s, Watch_Dogs high 70s/80s, Titanfall mid-80s. I think all of these were titles where the publishers were hoping that, like last gen, they'd be in the high-80s/low-90s ranges. In Watch_Dogs' case, the game was delayed substantially to retool it after bad mock reviews and other discouraging feedback. So it's possible we are entering into an era where, yes, the "med school students" of gaming are getting 70s.
It's also not something that dooms Ubisoft. I understand that Watch_Dogs has done quite well over time. Even AC1 did not perform to expectations critically, but it laid a foundation for iteration later which paid off with the huge growth and success of AC as a series.
Mordor, on the other hand, has gotta be a huge win for WB. Arkham Asylum was an unexpected smash hit and Mordor's high-80s MC is tracking much closer to AA than anything else. I think like AA, Borderlands, and BioShock (all unexpected breakout hits from last gen), Mordor's score/WOM is going to carry it a long way.
Circling back: Yes, 76 is a "bad" MC for AC:U, and yes it's pretty clearly lower than what Ubisoft or Ubisoft investors expected or hoped for. That's possible even if on the overall it's not a bad MC. It's not the end of the world.